startup: always wait for UI with --embed, unless --headless also is supplied

src/nvim/api/ui.c

@@ -58,6 +58,21 @@ void remote_ui_disconnect(uint64_t channel_id)
   xfree(ui);
 }
 
+/// Wait until ui has connected on stdio channel.
+void remote_ui_wait_for_attach(void)
+  FUNC_API_NOEXPORT
+{
+  Channel *channel = find_channel(CHAN_STDIO);
+  if (!channel) {
+    // this function should only be called in --embed mode, stdio channel
+    // can be assumed.
+    abort();
+  }
+
+  LOOP_PROCESS_EVENTS_UNTIL(&main_loop, channel->events, -1,
+                            pmap_has(uint64_t)(connected_uis, CHAN_STDIO));
+}
+
 void nvim_ui_attach(uint64_t channel_id, Integer width, Integer height,
                     Dictionary options, Error *err)
   FUNC_API_SINCE(1) FUNC_API_REMOTE_ONLY